In the first paragraph of Chapter 2, Thomas established that in the case of composite substances essence clearly comprises both matter and form. Now he begins to work out in what way this is so.
Nor can it be said that essence signifies the relation between the matter and the form or something superadded to these, for then the essence would of necessity be an accident and extraneous to the thing, and the thing would not be known through its essence, contrary to what pertains to an essence.
Right. Both the matter and the form are necessary, or there’s no dog. If the essence were something that arose out of these, or were added to these, then it would be accidental, and the essence of the dog could be known apart from its essence, which is absurd.
Through the form, surely, which is the act of the matter, the matter is made a being in act and a certain kind of thing. Thus, something that supervenes does not give to the matter existence in act simply, but rather existence in act in a certain way, just as accidents do, as when whiteness makes something actually white. Hence, when such a form is acquired, we do not say that the thing is generated simply but only in a certain way.
A dog can have many forms at the same time. It can, indeed must, have the form of dogginess, or it isn’t a dog. But it can also have the form of manginess. Form makes “a being in act and a certain kind of thing,” but a form that supervenes, like manginess, doesn’t give existence to the dog. It merely makes an existing dog mangy. “Hence, when such a form is acquired, we do not say that the thing is generated simply but only in a certain way.”
Remember that for a composite being to be “generated” means that it has come to be, as when a new human being is conceived. This is the fullest sense. We can say that mange is generated in a dog “in a certain way” when it acquires the form of manginess; but this is clearly an analogical sense of the word “generated”. (I talked about how words can have a scale of analogical senses in DE&E Chapter 1 (d).